Evidence of non-adiabatic electron heating at quasi-perpendicular shocks using MMS data

Electrons are efficiently heated at Earth's quasi-perpendicular bow shock, however the relative contributions of adiabatic and non-adiabatic processes, and the precise mechanisms behind non-adiabatic heating, remain open questions. Using MMS observations, we investigate two types of adiabatic mechanisms, direct acceleration by electric fields and adiabatic compression. We perform a statistical analysis of shock adiabaticity drawing on a large database of MMS bow shock crossings. The combined adiabatic effects of direct acceleration and compression well describe many shock crossings; however, for some crossings, these alone cannot account for the observed electron heating. To address this discrepancy, we apply a model for magnetic pumping, a non-adiabatic heating mechanism that transfers energy from large-scale compressive magnetic fluctuations directly to particles via an interplay between pressure anisotropy and pitch-angle scattering. For selected crossings, this model quantitatively accounts for the excess observed heating, providing evidence that magnetic pumping contributes to electron energization at the bow shock.

This work is based upon work supported by NASA under award No. 80NSSC23K0088.